1. Field of the Invention
The present invention relates to a pacemaker having a pulse generator for delivering stimulation pulses to a patient's heart, a control unit for controlling the delivery of stimulation pulses from the pulse generator, and a sensor provided to measure a parameter related to cardiac output of the patient.
2. Description of the Prior Art
In multi-site stimulation the PV-interval between the occurrence of an intrinsic P-wave and the application of a cardiac stimulation pulse in the ventricle can be different from the AV-interval between consecutive cardiac stimulation pulses to the atrium and the ventricle, and the PA-interval between the occurrence of an intrinsic P-wave in the right atrium and the application of a cardiac stimulation pulse in the left atrium can be different from the AA-interval between consecutive cardiac stimulation pulses to the right and left atria. The term AV-delay means in the interval between the occurrence of a paced or an intrinsic P-wave and the application of a cardiac stimulation pulse to the ventricle of the heart, and AA-delay means the interval between the occurrence of a paced or an intrinsic P-wave in the right atrium and the application of a cardiac stimulation pulse to the left atrium of the heart.
In cardiac therapy there is a primary aim to stimulate the heart such that the natural manner of a heart's functioning is re-established as far as possible. The heart is then working with a minimum waste of energy.
It is well known that cardiac output of a human being is depending on the AV-delay, see e.g. Swedish patent application no. 001534-7, corresponding to published PCT Application WO 01/80947.
In this application experimental data obtained from an animal study illustrate the dependence of several cardiac performance parameters on the AV-delay (or PV-delay). A changed AV-delay has an instant effect on the stroke volume SV but can also be observed on the mixed or central venous pressure PvO2 after a circulatory delay. A maximum PvO2 response was in the animal studies observed 50–80 s after the AV change. Thereafter it declined due to autonomic compensation. Thus, PvO2 mirrors the left heart's performance and reflects the cardiac output and its transient response can be used to optimize the AV-delay according to the algorithm described therein. Simultaneously the parameters oxygen saturation SO2 and carbon dioxide CO2 can be used for this purpose. The carbon dioxide concentration CO2 will, however, decrease in the same degree as the oxygen concentration increases.
It is also known from e.g. U.S. Pat. No. 4,928,688 to pace both ventricles of a heart to produce simultaneous contraction of both ventricles, thereby assuring hemodynamic efficiency. In this U.S. patent only simultaneous pacing of both ventricles is proposed. In U.S. Pat. No. 5,540,727 optimization of the pacing by selective stimulation in both atria and both ventricles with certain interatrial AA-, VV-, and AV-delays is described.